avoid redundant function calls

[Projekt: Entwicklung - Neon][TicketNr.: 1022526][Bereitstellung erste Version 5.1 Customizing für Gremium]

entity/History_entity/entityfields/subject_details/valueProcess.js

@@ -2,6 +2,9 @@ import("system.vars");
 import("system.result");
 import("Keyword_lib");
 
-var kwdUtils;
-kwdUtils = new KeywordUtils();
-result.string(vars.get("$field.SUBJECT") + " (" + kwdUtils.getViewValue("HISTORY.MEDIUM", vars.get("$field.MEDIUM")) + ")");
\ No newline at end of file
+var kwdUtils, histMedium;
+histMedium = vars.get("$field.MEDIUM");
+if (histMedium){
+    kwdUtils = new KeywordUtils();
+    result.string(vars.get("$field.SUBJECT") + " (" + kwdUtils.getViewValue("HISTORY.MEDIUM", histMedium) + ")");
+}
\ No newline at end of file